Proteases are important biomarkers for many pathologies and biological processes and are often targets of chemotherapeutics. Magnetic Resonance Imaging (MRI) is an outstanding diagnostic tool for identifying suspicious lesions, but suffers from many false-positive results due to a lack of specificity for pathological tissues versus normal tissues. The development of MRI contrast agents that can detect protease biomarkers of metastatic tumors or the response to anti-tumor therapies would greatly improve the specificity of MRI for detecting and evaluating cancer. We have developed a fundamentally new type of protease-responsive MRI contrast agent that is detected through PARAmagnetic Chemical Exchange Saturation Transfer (PARACEST). This mechanism allows each PARACEST contrast agent to be selectively detected, so that multiple PARACEST contrast agents may be simultaneously detected during a single MRI scan session, such as a protease-unresponsive PARACEST contrast agent that can serve as a `control'to account for the pharmacokinetics of the agents. Furthermore, we are the first research team to demonstrate the detection of PARACEST contrast agents within in vivo animal models. We will combine our accomplishments with PARACEST contrast agents to detect multiple enzyme-responsive PARACEST contrast agents within in vivo animal models. More specifically, we will design and apply PARACEST contrast agents to detect caspase-3 during TRAIL-induced tumor cell apoptosis in an in vivo mouse model of MCF-7c3 human mammary carcinoma. We will also design and apply PARACEST contrast agents to simultaneously detect cathepsin B and urokinase Plasminogen Activator in an in vivo mouse model of MCF-7 and MBA-MB-231 mammary carcinoma. The successful achievement of our specific aims will result in new platform technology that can be immediately used during many pre-clinical studies, and that can be used as a foundation for further studies to gain clinical approval for this new type of diagnostic molecular imaging agent to detect protease biomarkers in cancer and other pathological tissues. Narrative: A fundamentally new type of protease-responsive MRI contrast agent has been developed that is detected through PARAmagnetic Chemical Exchange Saturation Transfer (PARACEST). Furthermore, PARACEST contrast agents can be detected within in vivo animal models. This research application will combine these accomplishments with PARACEST contrast agents to detect multiple enzyme-responsive PARACEST contrast agents within in vivo animal models.